Summary of Significant Accounting Policies Property, Plant and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
PWGS | Minimum
Property, plant, and equipment
Estimated useful life	10 years
PWGS | Maximum
Property, plant, and equipment
Estimated useful life	45 years
ERGS | Minimum
Property, plant, and equipment
Estimated useful life	10 years
ERGS | Maximum
Property, plant, and equipment
Estimated useful life	55 years
Software | Minimum
Property, plant, and equipment
Estimated useful life	3 years
Software | Maximum
Property, plant, and equipment
Estimated useful life	15 years
WE
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	3.18%	2.95%	3.00%
WPS
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.50%	2.55%	2.58%
WG
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.30%	2.30%	2.34%
UMERC
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.50%	2.46%
PGL
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	3.25%	3.29%	3.31%
NSG
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.45%	2.43%	2.44%
MERC
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	1.95%	2.51%	2.53%
Reduction in Depreciation Expense Related to Depreciation Study	$ 1.4
MGU
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.61%	2.61%	2.63%